Temperature-sensitive elements are known that allow thermal energy to be converted into mechanical energy, especially by virtue of structures having a very high aptitude to deform under the effect of temperature. It may for example be a question of bimetallic strips (or bimetals). A bimetal comprises flexible strips (or membranes) of two different metal alloys with different thermal expansion coefficients, which strips are soldered or adhesively bonded face-to-face one against the other.
Below, Ksup and Kinf respectively designate the thermal coefficients of the membranes of the bimetal, where Ksup>Kinf, whereas MKsup and MKinf designate the corresponding membranes, referred to as the high-expansion membrane and the low-expansion membrane, respectively.
When a bimetal is heated, the high-expansion membrane MKsup tends to expand more than the low-expansion membrane MKinf, and the bimetal curves with a radius of curvature in the direction membrane MKsup toward membrane MKinf.
In contrast, when a bimetal is cooled, the high-expansion membrane MKsup tends to contract more than the low-expansion membrane MKinf, and the bimetal curves with a radius of curvature in the direction membrane MKinf toward membrane MKsup.
Thus, if the bimetal is repeatedly heated and cooled, its curvature reverses the same number of times. The reversals in curvature occur abruptly, the bimetal snapping from a first stable position to a second stable position; “bistability” is thus spoken of.
Thus, when a bimetal is placed between a heat source and a cold source, it oscillates from one stable position to the other cyclically. Such a cycle is illustrated in FIG. 1.
The curve H represents the variation in the vertical position Z of the middle of the curvature of the bimetal relative to a median position as a function of temperature T, and forms a hysteresis cycle. When the bimetal is heated from a first stable position 11, the curve remains substantially constant until a first snap-transition temperature Tc1, shown at the point 12.
At the temperature Tc1, passage from the point 12 to the point 13 leads to an abrupt reversal in the curvature. At the point 13, the bimetal is then in its second stable position.
Passage from the point 13 to the point 14 corresponds to a decrease in the temperature of the bimetal, the curvature remaining substantially constant to the point 14, at a second snap-transition temperature Tc2.
At the temperature Tc2, the passage from the point 14 to the point 11 leads to an abrupt reversal in the curvature. At the point 11, the bimetal has returned to its first stable position.
On each passage from one stable position to the other, curvature decreases then increases. The decrease in the curvature of the arc formed by the bimetal naturally leads to an elongation of the chord, and a resultant longitudinal force is thus generated at the ends of the bimetal on each snap transition.
Devices are also known for converting mechanical energy into electrical power, for example implementing the properties of piezoelectric materials. A piezoelectric material is a material that, when it is subjected to a mechanical deformation, generates an electrical voltage.
In the prior art, systems are known that combine a bimetal and a piezoelectric element in order to convert thermal energy into electrical power. The bimetal is generally placed between a heat source and a cold source and alternates from one curvature to the other cyclically, and the piezoelectric element is generally under tensile stress and especially of the type designated “3-1” in the art.
Such a system is especially described in patent application United States Patent Publication No. 2015/0115769 A1 (incorporated by reference), in which the piezoelectric element is a cantilever fastened to the bimetal. The suspended piezoelectric element is stimulated to vibrate when the bimetal changes position.
Another prior-art system has a piezoelectric element placed against one of the temperature sources. During a snap transition, some of the kinetic energy of the bimetal is transferred to the piezoelectric element by mechanical shock.
Current embodiments have many drawbacks, in particular: their mechanical and thermal transfer efficiencies are limited; their piezoelectric efficiency is undesirably affected by temperature; thermal energy diffuses into the piezoelectric element; their working frequency is low; mechanical energy is lost to the holder; or their structures may even lead to problems positioning and encapsulating elements of the system.